The present invention relates generally to the field of inhibit circuits for radios and more particularly to noise sensitive inhibiting circuits for FM stereo radios having a stereo indicating light.
Most FM (frequency modulated) stereo radios produce an audible noise hiss whenever the radio is not tuned to a transmitting FM channel frequency. The stereo indicating light is normally controlled by a stereo pilot detector which detects the presence of a 19kHz pilot (carrier) signal in the output signal of an FM detector. When the radio is not tuned to a station, the output of the FM detector is band limited noise. This noise results in an audible hiss at the speakers, but also can cause the pilot detector to falsely indicate the presence of the 19kHz carrier signal. This can therefore result in the flickering on and off of the stereo indicating light whenever the radio is not tuned to a station, thus annoying and misleading a person who is trying to tune the radio to a stereo station.
In some radio receivers, a mute signal is developed which is proportional to the magnitude of the IF (intermediate frequency) signal received by the FM detector. This mute signal is then used to inhibit (squelch) the output of the FM detector. This results in the positive inhibiting of the stereo indicating light as well as the total quieting of the radio speakers. In many applications, such as for FM stereo radios used in automobiles, such a result is not desirable since there is no audible indication given to the driver of the vehicle which alerts him to the fact that his radio is on. Thus these prior systems do not provide any audible indication that the radio is on whenever the stereo indicating light is inhibited because no FM signal is currently being received. Therefore the driver may falsely believe that his radio is inoperative (broken) or inadvertently leave his radio on and unnecessarily drain power from the electrical system of his car.